Voltage controlled oscillators (VCOs) are commonly employed in a variety of applications, including communication and timing circuitry. In particular, VCOs are commonly used in phase-locked loop (PLL) control systems. Functionally, a VCO may be viewed as a circuit that seeks to transform an input control voltage signal to an output voltage signal having a desired frequency.
In this case, following a frequency division of the output voltage signal, a phase/frequency detector is normally used to compare an output signal of an oscillator with a reference signal, and a loop filter is used to tune the VCO in a manner dependent on the phase/frequency comparison such that the output signal “matches” the reference signal. Such PLLs are usually used to synthesize signals at a desired frequency or, for example, to recover a clock signal from a data stream. PLLs can also be advantageously used in mobile radio for the purposes of signal modulation.
However, having the reference signal and the feedback signal close to one another may lead to long frequency locking time. To that end, it may be desired to provide an improved digital PLL.